Sublines of transplantable rat adenocarcinoma MTW9 have been developed which differ in content of estradiol and prolactin receptors, oophorectomy response, and growth rate. These are being studied in serial transplant and in culture for genetic stability, and growth control by hormones. The regulation of receptor content and nuclear transport of these sublines are being studied in terms of hormonal environment in an effort to explain why some tumors that possess estradiol receptor do not regress after oophorectomy. We have developed techniques which may distinguish receptor sites apparently homogeneous by Scatchard analysis. They are being used in measuring receptor sites for estradiol and prolactin in tumors growing under different hormonal conditions. MTW9 grows in only 20% of hosts with a normal serum prolactin but increased environmental temperature increases the number of tumors which grow out. MTW9-MtT (MTW9 growing in a rat with coimplanted mammosomatotropic MtT) does not regress after oophorectomy. The factor inhibiting oophorectomy regresssion is not prolactin or growth hormone. We are attempting to identify it.